Manufacture of fuel tanks



Jan. 28, 1947.

K. L. EDGAR' E'rAL MANUFACTURE 0F FUEL TANKS Filed May 19, 1945 4Sheets-Sheet 2 m A H Jan. 28, 1947. K. L. @GAR E'T 2,415,032

MANUFACTURE OF FUEL TANKS A Filed Mayl 19, 1945 4 Sheets-Sheet 3 KENNETHL. EDGAR AND HARRY H. GREGG Jan. 28, 1947. K, 1 EDGAR E1- AL 2,415,032

MANUFACTURE 0F FUEL TANKS Filed May 19, 1945 4 Sheets-Sheet 4 3mm/Mw KENN ET H L. EDGAR AND HARRY H.GI2EGG` Patented Jan. 28, 1947 UNITED-STATES PATENT OFFICE MANUFACTURE OF FUEL TANKS Kenneth L. Edgar andHarry H. Gregg, Akron, Ohio, assignors to The Firestone Tire & RubberCompany, Akron, Ohio, a corporation of Ohio Application May 19, 1945,Serial No. '594,671

(Cl. 15d-43.5)

2 Claims.

This invention relates to the fabrication of supplemental bullet-sealingfuel tanks for aircraft which may be jettisoned or dropped when empty.More particularly, it relates to the manufacturing of such tanks whichare of skin-stressed monocoque design.`

Jettison fuel tanks of the type here conteniplated are intended forequipping fighter aircraft for long-distance lighting and reconnaissancepurposes, and the tanks are made self-sealing so that they may becarried through fla-k and gunfire. They are to be supported by bombracks or the like mounted on the craft. l

The fuel tanks to which this invention relates employ the minimum ofmetal, are of skinstressed monocoque design, and require n bulkheads orother internal supporting structures. They are sufficiently strong andso constructed and united to the plane as to prevent distortion orswaying in flight. The various advantages of fabricating the tanks ashere described will be more evident as the description proceeds;

In the accompanying drawings:

Fig. l. is a fragmentary detail plan View of thev fuel tank showing themeans for fastening it to a plane; l

Fig.Y 2 is a section on the line 2--2 of Fig. l; Fig. 3 is a section onthe line 3-3 of Fig. 1; Fig. 4 is a section on the line 4-4 of Fig. 3;Figs. 5-9 illustrate the fuel tank in perspective at different stages inthe construction of the outer skin of the tank, Fig. 5 illustrating thebullet-'sealing tank or inner liner .after application of the canopyreinforce, Fig. 6 illustrating the inner liner with the reinforce turnedin in preparation for the application of the supporting plywood hoops,Fig. 'l' `the application of the hoops and first Vfabric coatings, andFigs. 8 and 9 the application of further coatings;

Fig. 10 is a detail in perspective showing the method of applying thecanopy reinforce;

Fig. 11 is a detail in section showing the position of the metal backingplate;

Fig. `12 is a `detail plan view of the underside of the tank showing theapplication of an adhesive ply around the fuel drain;

Fig. 13 is a sectional detail of the structure around a fitting; and

Fig. 14 is a conventional showing of the location of the seams betweenthe successive longitudinal plies.

To facilitate the understanding of the method of fabrication which is`the subject of this invention, the structure of the tank as a wholewill first be described with particular reference to Figs. 1-4, andthereafter the method of fabrication will be described in detail.

The fuel tanks are designed to be suspended from the bottom ofthe wingsof a plane between the fuselage andthe engine nacelles. Each `fuel `2tank is releasably suspended from a wing by means such las a bombshackle (not shown) which is preferably enclosed by fairing which servesto reduce 'wind resistance. For the same reason the fuel tanks are ofstreamline shape with substantially uninterrupted surface. i

The fuel tank is of monocoque design and cornprises an internalbullet-sealing lining or cell l? and a rigid, external, nonmetallicshell or casing i8, which is fabricated as later explained in detail.Thelinlng `Il Ais composed of a plurality of plies of treated fabric andsealing composition with an inner facing of material that is resistantto the action of hydrocarbon fuels. Since the composition of the lining`is `not a part of the present invention, a `more detailed descriptionthereof will not be given. At the top of the tank, in the medial regionthereof, the lining i1 has a flattened region 20, Fig. 3,I in which islocated a manhole or access opening 2|, the latter being rectangular inshape and having a metal rein forcing element 22 of similar shape builtinto its structure. The manhole 2| is closed by a cover 23 that iscomposed of composition similar to that employed in the remainder of thecell l1 and which has a reinforcing of metal in the margin thereof, saidcover being secured in place Iby a plurality of cap screws 24 thatextend through said cover and are threaded `into the reinforcing 22around the manhole. Adjacent its rear end the manhole cover 23 is formedwith a marginally reinforced aperture 25 through which extends a pipe 26that is utilized in withdrawing fuel from the tank. The access opening2| is large enough to enable a small vman to enter the tank for anypurpose that may arise.

Positioned at the top of the cell ll and covering a substantial regionthereof, including part of the flattened region 20, is a canopy 28 thatis composed of plywood. Said canopy is of oval shape in plan and islongitudinally and transversely arcuate in section so as to conform tothe general contour of the tank. Circumscribing the cell I1 are aplurality of girth straps or hoops 29, 29, herein shown as four innumber. The straps 29 are composed of plywood and have skived ends vthatare secured to complementally l skived portions on the margin of thecanopy 28 by `means of wood screws 3B and glue, as shown in Fig. 1. Thecanopy has a relatively large central opening 3| that is located overthe flattened medial region '20 of the cell l1, thel shape of saidopening being somewhat 'irregular, as will be apparent from Fig. 1. `Themargin of the opening 3| is reinforced by a metal backing plate 32attached to the under or concave side thereof, said backing plate formedwith a longitudinal series of apertures therethrough for a purposepresently to be explained.

The shell Aor casing I8 of the tank is composed of plies of fabricimpregnated and bonded to each other with a suitable heat-hardenedplastic composition. Of the plastic compositions Afound suitable for thepurpose may be mentioned one cornposed of urea-formaldehyde and Buna Ssynthetic rubber, and another one composed of phenolic resin and Buna S.rubber-like copolymer of butadiene and styrene.) When the rst-mentionedcomposition is used, the cell or lining I1 must be vulcanized before theshell or casing I8 is constructed thereon since said composition isinjured by the higher tem.- perature required to vulcanize the cell Whenthe second composition is used in the shell, both lining and shell maybevulcanized at the same time if desired. The vulcanized shell is denseand hard and has substantial strength. It is to be understood that theplastic composition may be tacky so that plies coated with it willadhere to one another, or the plastic may be made tacky by theapplication of a liquid thereto.

The shell or casing I8 covers all of the cell l1 including the hoops 28and all of the canopy 28 thereon except for an area adjacent the marginof the opening 3I therein, the margin of the shell about said openingbeing designated 34. The strips 35 (Fig. ll) unite the hoops 29 to thecell I1 and shell I8, as will be explained in greater detail in whatfollows.

Mounted within the opening 3| of the canopy is a deep metal pan 31 thathas an integral marginal flange 38 that rests upon the exposed portionof the canopy within the shell opening dened by the margin 34, said panflange being secured to the canopy by a plurality of Cap screws 38, 38that extend through the flange and canopy and are threaded into nuts 66welded to the bottom of the metal backing plate 32 of the canopy. A thinmetal cover plate or fairing 48 fitting closely around the pan 31overlies the flange 38 and margin 34 of the shell opening and concealsthe cap screws 39. The fairing is secured in place by countersunk screws4I that are threaded into the flange 38. Welded and/or riveted in thepan 31 at two spaced apart points are respective brackets that extendcompletely across the same, each bracket comprising a central web 42 andreinforcing webs 43, 43 at opposite sides thereof. Each of said centralwebs is formed with an aperture 44 therein, and the tops of the webs 43are centrally notched at 45 substantially as deep as the bottom of saidaperture. Between the said brackets the pan is reinforced by alongitudinally extending inverted channel 46 welded to the bottom of thepan. Said brackets have end walls 55 that extend upwardly above the topmargin of the pan, the upper margin of each of said end walls beingformed with two oblique surfaces meeting at a point, one of said obliquesurfaces, designated 41, being provided for a purpose presently to beexplained. The bottom of the pan is formed with a suitable aperture toenable the fuel pipe 26 to extend therethrough,

The fuel tank is suspended from an aircraft through the agency of bombshackles that engage the webs 42 of the brackets in pan 31 and extendthrough the apertures 44 therein. The bomb. shackles are of theconventional type used for releasably carrying bombs. The fairing thatsurrounds the bomb shackles is of the same body contour as the pan 31,and is arranged tobe received within the latter when the fuel tank issuspended from the bomb shackles. The bottom margin of the fairing isshaped to conform to the contour of the bottom of the pan 31 and is(Buna S is a synthetic notched on each side thereof to span the webbedbrackets within the pan. When the fuel tank is suspended from the bombshackles, the end walls of the said brackets are disposed exteriorly ofthe said fairing, the oblique marginal surfaces 41 of said end wallsengaging complementally oblique faces formed on opposite ends ofrespective bearing strips that are mounted on opposite sides of thefairing structure. The arrangement assures accurate registry of the fueltank with its supporting structure and provides stability by preventingside sway and longitudinal oscillation or tilting of the fuel tank.

Preferably some of the space between the canopy 28 and the flattenedregion 28 of the lining or cell I1 at each side of pan 31 is lled withsuitably shaped blocks of wood 58, which may be of balsa because of itslight weight.

The fuel tank will hold its shape in flight and under all conditions ofloading, yet employs but little metal in its construction. The entireload of the tank is carried by the stressed skin or shell I8 thereof,and the presence of the rigid plywood canopy 28 which extends about 30per cent of the area of the top of the tank insures adequatedistribution of the load. Furthermore, the arrangement for attaching thetank to an aircraft prevents swaying of the tank during night. Thelocation of the pan 31 within the tank and substantially below thecanopy 28 protects the pan against gunfire such as might result inloosening or separating the fuel tank from the airplane. Also, thepresence of the webs 43 at opposite sides of the central webs 42, thatare engaged by the shackles, additionally protect the webs 42 fromgunfire.

Manufacture of nonmetallz'c shell The first step in the manufacture ofthe nonmetallic shell I8 is to prepare the bullet-sealing tank for theplacement of the canopy. This is done by placing the so-called canopyskirt reinforce 88 in place in the canopy recess 6I, which is formedaround the outer edge of the flattened region 28 of the bullet-sealingcell or liner I1. Fig. l0 is a detail showing the manner of application.The inside edge of the reinforce for a distance of at least about 2inches is cemented in the recess. This leaves an uncemented flap 62which is later folded back over the canopy as shown in Fig. 7. This flapties the plywood canopy in place and provides a union between the canopy28 and the shell I8 and the lining I1. It prevents steam used in thesubsequent vulcanization from penetrating between the outer nonmetallicshell I8 and the inner bullet-sealing construction I1. The fabricV usedfor making the reinforce, before being cut to size, is treated withrubber or other curable adhesive which on subsequent vulcanization willunite with the bulletsealing liner I1.

The plywood canopy 28 is now prepared for placement The underside of thecanopy is coated with a ply of fabric which is treated with a suitableplastic material which is preferably the same as the plastic used fortreating the other plies of the nonmetallic shell. It protects thecanopy from the steam used in the subsequent vulcanization and serves tostrengthen the bond between the canopy and the reinforce,

This ply 65 is trimmed so that it does not overlap the metal backingplate 32. The nuts 66 (Fig. 11) are Welded to the underside of the metalbacking plate in line with perforations inthe of the canopy reinforce issimilarly treated. The

canopy is then put in place and cemented in position with a suitableoil-resistant cement, such as a solution of a rubber-like copolymer ofbutadiene and acrylonitrile.

The edge of the skirt or canopy reinforce is cut with dutchmen t3 whichshould not be over an inch and a half deep so that the skirt t2 willliefflat against the upper surface of the canopy. The flap E52 willoverlie the upper surface of the canopy for a distance of about Zinchesfrom the edge of the canopy, except adjacent the depressions 69 in thelining which are to receive the plywood bands or hoops 29, and here at1U the width of the overlap should not be greater than 1/2 inch.

Strips of bonding material 35, somewhat wider than the hoops 29, arethen put in place as shown in Fig. 6, These strips are of fabric coatedon the underside with a curable adhesive, such as rubber or the like,which will form a good bond with the liner l1. The strips 35 arepreferably cut on a Ll-degree bias. The ends of the strips are trimmedflush with the canopy reinforce (it.

When a bullet-sealing tank is pierced by a bult, the wall of the tankquickly returns to its original position to effect the sealing of thehole.

It is important in forming the outer shell of the tank that there be aslittle interference as possible with such return of the bullet-sealingcell to its original location. if the outer nonmetallic skin werecemented to the outer surface of the bullet-sealing cell, the rigidityof the outer skin would interfere with this self-sealing property of thecell. It might, therefore, seem desirable to merely fit the outer shellaround the bullet-sealing tank without any adherence between the two.However, with such a construction the bulletsealing liner sags away fromthe outer shell at the top and on the ends when the tank is subjected togravity loading due to diving or other plane maneuvers.

It has, therefore, been found desirable to secure adherence between theouter shell and the bullet-sealing tank in restricted areas. Suchadherence is desirable around the fittings and at the ends of the tank.This is shown in Fig. '7, which shows the fitting 1i to which the cap12, which covers the opening for filling the fuel into the tank, isfastened. It, likewise, shows the fitting 13 in which the drain opening(not shown) is provided. These fittings are preferably of oilresistantsynthetics, such as Buna N. The local adherence between thebullet-sealing liner I1 and the outer shell is obtained by the plies1li. These are coated on the undersurface with rubber or other curableadhesive which gives a good bond with the liner i1 on curing. Theirouter surface is brushed with a solution of a plastic, such as that usedon the other plies of the skin to produce adherence between them.

If the inner cell is cured before the outer skin, these plies 15 are putin place before this curing step is effected. Although they arepreferably placed over the U flange of the fittings 1I and 13, asillustrated in Fig. 13, they may, if preannees vA ply of nonadheringmaterial 16 of the same thickness as the plies 15 is placed over theremainder of the tank; i. e., the surface thereof not covered by theplies 15 and the strips 35. The ply 16 does not permanently adhere tothe bulletsealing cell but is coated on its outer surface with plasticadhesive so that it adheres to, and on vulcanization becomes a part of,the outer nonmetallic covering. The plies 15 and 16 are trimmed flushwith the edge of the canopy.

The cell is now prepared for placement of the plywood bands 2S bybrushing the outer surface of the strips with a plastic adhesive whichforms a strong bond with the plywood bands. The outer surface of thecanopy where the plywood bands are to be placed, is skived so that theouter surface of the bands will lie flush with the outer surface of thecanopy. The skiving is illustrated at 'i8 in Fig. 3. The canopy is thendrawn tight against the cell before placing the plywood bands 29 inplace. This is advantageously accomplished by encircling the canopy andtank with metal bands or straps supplied with turn buckles or the likefor drawing the canopy` tight against the lining l1. The underside ofeach of the plywood bands is then coated with a suitable adhesive, andthe bands are applied over the strips 35, and the end of each isfastened into the canopy by four wood screws 3l] (Fig. 1). The edge t?.of the canopy reinforce is then pressed to the upper surface of thecanopy. Any Void between the edge of the canopy and the shell is thensuitably filled with small strips of fabric treated with the curableadhesive.

The cell is now ready for the application of the outer fabric plies.These plies are composed of fabric treated with a curable plastic. Aspreviously suggested, this plastic may be a mixture of anurea-formaldehyde thermoset and Buna S synthetic rubber or may be amixture of a phenolic resin and Buna S. The plastic is compounded withsuitable vulcanizing ingredients. The cell and cano-py are alternatelycovered with longituferred, be laid over the liner l1 within the jaws ofthe U flanges of the fittings before these are closed. The plies 15advantageously cover the area around the fittings for a distance ofabout 6 inches.

dinal and transverse plies. It is advantageous to 'd first applylongitudinal plies of fabric, and these are indicated by the referencenumeral 8i) in Fig. 8. These constitute the second ply of the shell.There are five of these pieces extending from one end of the cell to theother. The joints located at the positions 2 of Fig. 14 are all buttspliced. The fabric is advantageously out on a bias of about 20 degrees.

Transverse plies are then applied circumferenr tially as indicated inFig. 9. The nose piece 82 is formed from a flat disk which is shown inplace ready for application in Fig. 8. The tail piece is formed of apiece t3 having the contour illustrated in Fig. 8. After placing, theedges of the piece 83 are trimmed so that the piece 83 lies flat. Theintermediate pieces are laid as shownwith butt splices at the seams.There is no bias to these pieces which constitute the third ply.

The filler fitting 1| and drain fitting 13 are in depressed areas sothat no bolts or the like protrude from the surface of the cell tointerfere with its streamlined contour. Fig. 13 shows how, after thedepressed areas have been covered with the first three plies, fillerpieces Bti of fabric coated with the curable adhesive areused to reducethe size of the depressed areas around the drain and filler openings.Fig. 13, likewise, shows a ring 38 embedded in the fitting which hasthreaded openings therein to receive bolts used to bolt the cap 12 (Fig.'7) in place over the opening 89.

After placing the iiller pieces 85 in place, additional coverings areapplied to the entire surface of-the tank. There are altogether sevenplies in the preferred nonmetallic shell structure described herein andshown in the drawings. Each of these plies (except the rst which istreated as described) is coated with a curable plastic so that the pliesare all united into a unitary shell on subsequent vulcanization. Thefirst ply which is a composite, some of which adheres to the lining andsome of which does not adhere, is sho-wn in Fig. 7. Fig. 8 shows thesecond ply of covering material. The third ply is illustrated in Fig.The fourth ply is of the type illustrated in Fig. 8, preferably cut on aIlo-degree bias, the longitudinal'splices being located Several inchesto one side vof the second ply splices illustrated in 8. The arrangementof the splices in the longitudinal plies is sho-wn in a conventionalmanner in. Fig. 14, the numerals indicating the number the` ply theseams of which are located at the respective positions indicated by thelocations of the respective numbers. For example, at the top of thefigure are the numerals 5, 4, 2, and l. These indicate the location ofthe splices in the sixth, fourth, second, and seventh plies,respectively.

The fifth ply is of the type illustrated in Fig. 9, but the capscovering the ends extend back a couple of inches further than the capsand 83, respectively, and the location of the intermediate splices isstaggered somewhat. There is no bias to the pieces in this ply.

The sixth ply is built up of longitudinal pieces of material similar tothose illustrated in Fig. S. The fabric is preferably cut on a 20-degreebias. The splices are staggered somewhat from the splices of the priorplies as indicated in Fig. 14.

The sixth ply is covered by a seventh ply, the pieces of which also runlongitudinally in the manner shown in Fig. 8. The pieces are cut on aZO-degree bias, and the splices are staggered somewhat from the splicesof the preceding plies as indicated in Fig. 14.

Each of the splices in the last ply of material is covered by areinforce coated with curable plastic which is carefully pressed orstitched down to prevent any separation. The edges around the fittingsare carefully calked With cement of a suitable oil-resistant plasticmaterial. and these are allowed to thoroughly dry.

Constructed in this manner with each of the plies carefully spread outso as to t tight against f the previously laid ply of material, thestrong, serviceable, nonmetallic skin covering is produced whichtogether with the metal backing plate and the anchoring means takes upthe stresses suffered bythe fuel tank when suspended from an airplane ascontemplated,

The nonmetallic covering must be cured to unite the various plies andmake a unitary cover. If the covering plies have incorporated Itherein amixture of an urea-formaldehyde resin and uncured Buna S, two curingoperations will be required. The bullet-sealing tank with the plies 15and strips 35 in place will be separately cured before the skin-coveringplies are applied. This cure will be effected at a temperature in theneighborhood of 300 F. Then after the outside cover plies have all beenput in place, the resin in these plies will be heated to about 240 F.for curing. A combined steam-air cure has been found desirable, usingsteam at about pounds pressure. A longer time will be required to curewith air alone. The cell I'l is built over` a frangible core (notshown), which core will preferably be allowed to remain until after theoutside cover has been completed and cured although in rare instances itmay be removed from the bullet-sealing cell before the outside plies andcanopy are placed. A papier-mache or plaster core has been foundsuitable.

If a phenolic resin which requires a higher temperature for curing isincorporated in the plies of the outer shell-e. g., a mixture of a'phenolic resin and uncured Buna S-a single cure is all that isrequired. The outer covering will then be built around the fuel-celllining while .the lining is still in the green state and on the core. Asuitable cure might, for example, be 300 F. for 10 minutes followed byturning oi the heat and allowing the temperature to drop for 17 minutes,at the end of which time it will be about 275 F. This temperature of 275F. should be maintained for minutes.

After curing, regardless of which curing method is employed, the entireassemblage is sprayed with water to cool it. The core is then removedfrom the interior ofthe finished tank, and the interior of the tank iscarefully cleaned out. The tank is .then ready for completion byinsertion of the pan 31, etc.

Although the use of the pan 3l' for coupling with bomb shackles has beenillustrated, it is obvous that any method of coupling the tank to asupport on the plane may be used. Modification may be resorted towithout departing from the spirit of the invention or the scope thereofas defined by the appended claims.

What; we claim is:

1. The method of fabricating a jettisonable fuel tank with abullet-sealing lining which comprises curing the lining with flattened-top surrounded by a recess for reception of a canopy and encirclingdepressions in the walls thereof for the reception of reinforcing hoops,afhxing a reinforcing strip in the recess with a flap extendingoutwardly therefrom, placing the canopy on the strip, turning the iiapback over the edges of the canopy, putting the reinforcing hoops inplace and fastening them to the canopy, and then covering the hoops andmost of the canopy with plies of fabric treated with a curable plasticand curing.

2. The method of fabricating a jettisonable fuel tank with a curablebullet-sealing lining with a flattened top surrounded by a recess forreception of a canopy and encircling depressions in the walls thereoffor the reception of reinforcing hoops, which comprises aiiixing areinforcing strip in the recess with a flap extending outwardlytherefrom, placing the canopy on the strip, turning the flap back overthe edges of the canopy, putting the reinforcing hoops in place andfastening them t0 the canopy, covering the lining and the hoops and mostof the canopy with plies of fabric treated with a curable plastic, andthen simultaneously curing the lining and plies.

KENNETH L. EDGAR. HARRY H. GREGG.

